Long before seat belts or common sense were particularly widespread, my family made annual trips to New York in our 1963 Valiant station wagon. Mom and Dad took the front seat, my infant sister sat in my mother’s lap and my brother and I had what we called “the wayback” all to ourselves.

In the wayback, we’d lounge around doing puzzles, reading comics and counting license plates. Eventually we’d fight. When our fight had finally escalated to the point of tears, our mother would turn around to chastise us, and my brother and I would start to plead our cases. “But he hit me first,” one of us would say, to which the other would inevitably add, “But he hit me harder.”

It turns out that my brother and I were not alone in believing that these two claims can get a puncher off the hook. In virtually every human society, “He hit me first” provides an acceptable rationale for doing that which is otherwise forbidden. Both civil and religious law provide long lists of behaviors that are illegal or immoral — unless they are responses in kind, in which case they are perfectly fine.

After all, it is wrong to punch anyone except a puncher, and our language even has special words — like “retaliation” and “retribution” and “revenge” — whose common prefix is meant to remind us that a punch thrown second is legally and morally different than a punch thrown first. That’s why participants in every one of the globe’s intractable conflicts — from Ireland to the Middle East — offer the even-numberedness of their punches as grounds for exculpation.

The problem with the principle of even-numberedness is that people count differently. Every action has a cause and a consequence: something that led to it and something that followed from it. But research shows that while people think of their own actions as the consequences of what came before, they think of other people’s actions as the causes of what came later.

In a study conducted by William Swann and colleagues at the University of Texas, pairs of volunteers played the roles of world leaders who were trying to decide whether to initiate a nuclear strike. The first volunteer was asked to make an opening statement, the second volunteer was asked to respond, the first volunteer was asked to respond to the second, and so on. At the end of the conversation, the volunteers were shown several of the statements that had been made and were asked to recall what had been said just before and just after each of them.

The results revealed an intriguing asymmetry: When volunteers were shown one of their own statements, they naturally remembered what had led them to say it. But when they were shown one of their conversation partner’s statements, they naturally remembered how they had responded to it. In other words, volunteers remembered the causes of their own statements and the consequences of their partner’s statements.

What seems like a grossly self-serving pattern of remembering is actually the product of two innocent facts. First, because our senses point outward, we can observe other people’s actions but not our own. Second, because mental life is a private affair, we can observe our own thoughts but not the thoughts of others. Together, these facts suggest that our reasons for punching will always be more salient to us than the punches themselves — but that the opposite will be true of other people’s reasons and other people’s punches.

Examples aren’t hard to come by. Shiites seek revenge on Sunnis for the revenge they sought on Shiites; Irish Catholics retaliate against the Protestants who retaliated against them; and since 1948, it’s hard to think of any partisan in the Middle East who has done anything but play defense. In each of these instances, people on one side claim that they are merely responding to provocation and dismiss the other side’s identical claim as disingenuous spin. But research suggests that these claims reflect genuinely different perceptions of the same bloody conversation.

If the first principle of legitimate punching is that punches must be even-numbered, the second principle is that an even-numbered punch may be no more forceful than the odd-numbered punch that preceded it. Legitimate retribution is meant to restore balance, and thus an eye for an eye is fair, but an eye for an eyelash is not. When the European Union condemned Israel for bombing Lebanon in retaliation for the kidnapping of two Israeli soldiers, it did not question Israel’s right to respond, but rather, its “disproportionate use of force.” It is O.K. to hit back, just not too hard.

Research shows that people have as much trouble applying the second principle as the first. In a study conducted by Sukhwinder Shergill and colleagues at University College London, pairs of volunteers were hooked up to a mechanical device that allowed each of them to exert pressure on the other volunteer’s fingers.

The researcher began the game by exerting a fixed amount of pressure on the first volunteer’s finger. The first volunteer was then asked to exert precisely the same amount of pressure on the second volunteer’s finger. The second volunteer was then asked to exert the same amount of pressure on the first volunteer’s finger. And so on. The two volunteers took turns applying equal amounts of pressure to each other’s fingers while the researchers measured the actual amount of pressure they applied.

The results were striking. Although volunteers tried to respond to each other’s touches with equal force, they typically responded with about 40 percent more force than they had just experienced. Each time a volunteer was touched, he touched back harder, which led the other volunteer to touch back even harder. What began as a game of soft touches quickly became a game of moderate pokes and then hard prods, even though both volunteers were doing their level best to respond in kind.

Each volunteer was convinced that he was responding with equal force and that for some reason the other volunteer was escalating. Neither realized that the escalation was the natural byproduct of a neurological quirk that causes the pain we receive to seem more painful than the pain we produce, so we usually give more pain than we have received.

Research teaches us that our reasons and our pains are more palpable, more obvious and real, than are the reasons and pains of others. This leads to the escalation of mutual harm, to the illusion that others are solely responsible for it and to the belief that our actions are justifiable responses to theirs.

None of this is to deny the roles that hatred, intolerance, avarice and deceit play in human conflict. It is simply to say that basic principles of human psychology are important ingredients in this miserable stew. Until we learn to stop trusting everything our brains tell us about others — and to start trusting others themselves — there will continue to be tears and recriminations in the wayback.

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The next couple of parts of this series will look to situationist essays written in the immediate aftermath of the 9/11 terror.